In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic printing plates typically comprise one or more imageable layers applied over the hydrophilic surface of a substrate. The imageable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the element is considered as positive-working. Conversely, if the non-imaged regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital imaging has become increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers that image in response to signals from a digital copy of the image in a computer a platesetter. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to image the elements.
Thermal imaging has especially become important with digital imaging systems because of their stability to ambient light. The elements are designed to be sensitive to heat or infrared radiation and can be exposed using thermal heads or more usually, infrared laser diodes. Heat that is generated from this exposure can be used in a number of ways, for example, ablation to physical remove imaged areas, polymerization of photosensitive compositions, insolubilization by crosslinking polymers, rendering polymers alkaline solution soluble, decomposition, or coagulation of thermoplastic particles. Most of these imaging techniques require the use of alkaline developers to remove exposed (positive-working) or non-exposed (negative-working) regions of the imaged layer(s).
Thermally meltable or fusable particles having surface functional groups have been used in imageable elements as described for example, in U.S. Pat. No. 6,218,073 (Shimizu et al.), U.S. Pat. No. 6,509,133 (Watanabe et al.), and U.S. Pat. No. 6,627,380 (Saito et al.). Other meltable polymeric particles are described in U.S. Pat. No. 6,692,890 (Huang et al.).
Coalesceable thermoplastic polymeric particles dispersed within hydrophilic binders in imageable elements are described, for example, in U.S. Pat. No. 6,030,750 (Vermeersch et al.) and U.S. Pat. No. 6,110,644 (Vermeersch et al.).
Core-shell particles are used in imageable layers according to U.S. Pat. No. 5,609,980 (Matthews et al.) and coalesce upon thermal imaging. The shell of the particles is soluble or swellable in aqueous media.
EP 514,145A1 (Matthews et al.) describes thermally-sensitive imageable elements containing heat-softenable core-shell particles in the imaging layer. Such particles coalesce upon heating and the non-coalesced particles are removed using an alkaline developer. The shells of these particles are specifically non-water soluble at neutral pH 7.
A similar composition is described in EP 1,642,714A1 (Wilkinson et al.). Non-exposed particles are removed using a gum solution instead of an alkaline developer.
Copending and commonly assigned U.S. Ser. No. 12/017,366 (filed Jan. 22, 2008 by Jarek) describes the use of coalesceable core-shell polymeric particles in imageable elements.
Typically, a water-soluble IR dye or contrast dye is added to such particles in the coating formulation. The IR dye is responsible for heat conversion under IR radiation so that the particles coalesce and form an image. The contrast dye improves the color intensity that allows better quality control of exposed printing plates. The dyes build something like a matrix around the particles in the coating or they may fill the cavities among the particles.
There are several drawbacks of such dye additions:
a) For ideal coalescence, a complete melting up of the particle is required in order to form a smooth film. Under IR irradiation, the IR dye converts the heat at the particle surface from which it has to be transferred into the inner particle zones. This heat transfer takes time or consumes a lot of exposure energy.
b) The dyes are usually not chemically bonded or fixed in any other way with the particles and therefore can be readily extracted from the exposed and coalesced image particles during the development step or by press room chemicals (for example, blanket washes) during printing that can lead to a significant loss of color contrast.
c) Most contrast dyes, especially cyanine dyes, are relatively sensitive to oxidation that reduces the shelf life of printing plates. This can be seen from a color shift of the printing plates with increasing storage time from a greenish blue color to a brownish color tone.
d) As long as the contrast dyes are within the matrix of the particles, they can be regarded as an additive. Generally, any additives (in this case the dyes) in the coating except within the particles themselves diminish the coalescence of the particles. The higher the amount of these dyes, the lower the contact of the particles with each other or in other words the average distance between particles grows. Particles with reduced contact with each other show weaker coalescence which results in lower run length of the resulting printing plate. Further, if the additives are water-soluble, as it is the case for most contrast dyes, the coalesced polymer particles can be mechanically destabilized by extraction of the dyes, which again leads to shorter press run length.